Predicting intramammary infection status at drying off using indirect testing of milk samples

Randomized, noninferiority trial evaluating the efficacy of a novel teat sealant in pasture grazed dairy cows

The intramammary infusion at the end of lactation of a bismuth subnitrate internal teat sealant (ITS), with no antibiotic component has been shown to be an effective means of reducing new intramammary infections over the dry period. There has, however, been very few comparative studies between different brands of ITS under grazed pasture conditions. The objectives of this study were, therefore, to determine if a new bismuth subnitrate internal teal sealant (ShutOut, MSD Animal Health) was noninferior to Teatseal (Zoetis) regarding end-points such as (a) detection of the ITS product after calving, (b) clinical mastitis during the dry period and early lactation, and (c) subclinical mastitis at 30 to 60 d in milk. A total of 1,105 mixed-age cattle were enrolled across 2 farms comparing 2 ITS products for detection of the ITS at calving and prevention of clinical and subclinical mastitis. Both ITS contained 65% (2.6 g) bismuth salts emulsified in ≤ 1.4 g of mineral oil (ShutOut as investigational product, IVP; Teatseal as control product, CPT). At dry-off, treatment was allocated to every second cow. All cows met industry best practice criteria for using ITS treatment without antibiotics. Outcomes included detection of ITS at first stripping of the udder by the farmer, clinical mastitis (CM) from dry-off to 30 d following calving and subclinical mastitis at 30 to 60 d following calving. For ITS detection, a generalized mixed linear regression model was used to model the data, with clustering of quarters within cow accounted for by including cow as a random intercept. Clinical mastitis was analyzed at the cow-level using a Fisher's exact test, and SCC was modeled using a negative binomial distribution. The IVP was noninferior to the CPT for ITS detection following calving. There were 1344/1800 (71.5%) of quarters with ITS detection in the IVP in comparison to 1076/1604 (67.1%) of quarters in the CPT treated group. The quarter-level CM incidence risk was low (45 cases out 4,324 quarters; 1.04%). The overall cow-level CM risk was 4.1% (44/1081), with 20/540 (3.7%) cases in animals in the IVP group and 24/541 (4.4%) cases in animals in the CPT group. The IVP was noninferior to the CPT for cow-level mastitis incidence. The median SCC for all animals was 23,000 cells/mL, with a mean of 92,000 cells/mL. The back-transformed estimated marginal mean estimated SCC was 84,800 (95% CI 75,200-95,600) cells/mL for animals in the IVP group, and 98,800 (95% CI 87,600-111,300) cells/mL for animals in the CPT group. The IVP was, therefore, noninferior for all outcomes measured.

Bacteriological outcomes following random allocation to quarter-level selection based on California Mastitis Test score or cow-level allocation based on somatic cell count for dry cow therapy

Intramammary infusion of antimicrobials at the end of lactation (dry cow therapy) has been a cornerstone of mastitis management for many years. However, as only a proportion of cows are infected at this time, treating only those cows likely to be infected is an important strategy to reduce antimicrobial usage and minimize risk of emergence of antimicrobial resistance. Such an approach requires the ability to discriminate between cows and quarters likely to be infected and uninfected. This study compared assignment of cows or quarters to antimicrobial treatment at the end of lactation based on cow composite somatic cell count (SCC; i.e., all quarters of cows with a maximum SCC across lactation >200,000 cells/mL received an antimicrobial; n = 891 cows, SCC-group) or assignment to quarter-level treatment based on a quarter level California Mastitis Test (CMT) score ≥ trace (n = 884 cows; CMT-group) performed immediately before drying off. All quarters of all cows also received an infusion of a bismuth-based internal teat sealant. Milk samples were collected for microbiology following the last milking, and again within 4 d of calving. Clinical mastitis records from dry off to 30 d into the subsequent lactation were collected. Multilevel, multivariable models were used to assess the effect of assignment to antimicrobial treatment. At drying off, a total of 575 (8.8%) and 147 (2.3%) of the 6,528 quarters had a minor, and a major intramammary infection (IMI), respectively. At drying off, 2089/3270 (63.9%) and 883/3311 (26.7%) of quarters were treated with dry cow therapy in the CMT and SCC-groups, respectively. Apparent bacteriological cure proportion for any IMI was higher in quarters assigned to the CMT than the SCC-group (349/368 (0.95, 95% CI 0.92-0.97) versus 313/346 (0.90, 95% CI 0.87-0.93)). New IMI proportion was lower among quarters assigned to the CMT than SCC-group [101/3,212 (0.032, 95% CI 0.025-0.038) versus 142/3,232 (0.044, 95% CI 0.036-0.051)]. The prevalence of any IMI postcalving was lower in quarters assigned to the CMT than SCC-group [119/3,243 (0.037, 95% CI: 0.030-0.044) versus 173/3,265 (0.054, 95% CI: 0.045-0.062)]. There was no difference in incidence of clinical mastitis between treatment groups. The total mass of antimicrobials used was 63% higher in the CMT-group than in the SCC-group (3.47 versus 2.12 mg/kg of liveweight). Selection of quarters for antimicrobial treatment at the end of lactation based on CMT resulted in greater proportion undergoing bacteriological cure, reduced risk of any new IMI and reduced post calving prevalence of any IMI compared with selection of cows based on SCC. However, CMT-based selection resulted in higher antimicrobial use compared with SCC-based selection, and further research is required to analyze the cost benefit and impact on risk of antimicrobial resistance of these 2 strategies.

Effects of Selective Dry Cow Treatment on Intramammary Infection Risk after Calving, Cure Risk during the Dry Period, and Antibiotic Use at Drying-Off: A Systematic Review and Meta-Analysis of Current Literature (2000-2021)

The objectives of this paper were (i) to perform a systematic review of the literature over the last 21 yr and (ii) to evaluate the efficacy of selective dry cow treatment (SDCT) vs. blanket dry cow treatment (BDCT) in dairy cows regarding the risk of intramammary infection (IMI) after calving, new IMI risk after calving, cure risk during the dry period, and a reduction in antibiotic use at drying-off by meta-analysis. The systematic search was carried out using the databases PubMed, CAB Direct, and ScienceDirect. A meta-analytical assessment was performed for each outcome of interest using random-effects models, and the relative risk (RR) for IMI and cure or the pooled proportion for antibiotic use was calculated. The final number of included studies was n = 3 for IMI risk after calving and n = 5 for new IMI risk after calving, cure risk during the dry period, and antibiotic use. The RR levels for IMI (RR, 95% confidence interval [CI]: 1.02, 0.94-1.11; p = 0.592), new IMI (RR, 95% CI: 1.06, 0.94-1.20; p = 0.994), and cure (RR, 95% CI: 1.00, 0.97-1.02; p = 0.661) did not differ significantly between SDCT and BDCT. Substantial heterogeneity was observed between the trials regarding the pooled proportion of antibiotic use within the SDCT groups (I² = 97.7%; p < 0.001). This meta-analysis provides evidence that SDCT seems to be an adequate alternative to BDCT regarding udder health with a simultaneous reduction in antibiotic use. Limitations might arise because of the small number of studies included.

Evaluation of 4 predictive algorithms for intramammary infection status in late-lactation cows

The objective of this observational study was to compare 4 cow-level algorithms to predict cow-level intramammary infection (IMI) status (culture and MALDI-TOF) in late-lactation US dairy cows using standard measures of test performance. Secondary objectives were to estimate the likely effect of each algorithm, if used to guide selective dry cow therapy (SDCT), on dry cow antibiotic use in US dairy herds, and to investigate the importance of including clinical mastitis criteria in algorithm-guided SDCT. Cows (n = 1,594) from 56 US dairy herds were recruited as part of a previously published cross-sectional study of bedding management and IMI in late-lactation cows. Each herd was visited twice for sampling. At each farm visit, aseptic quarter-milk samples were collected from 20 cows approaching dry-off (>180 d pregnant), which were cultured using standard bacteriological methods and MALDI-TOF for identification of isolates. Quarter-level culture results were used to establish cow-level IMI status, which was considered the reference test in this study. Clinical mastitis records and Dairy Herd Improvement Association test-day somatic cell count data were extracted from herd records and used to perform cow-level risk assessments (low vs. high risk) using 4 algorithms that have been proposed for SDCT in New Zealand, the Netherlands, United Kingdom, and the United States. Agreement between aerobic culture (reference test; IMI vs. no-IMI) and algorithm status (high vs. low risk) was described using Cohen's kappa, test sensitivity, specificity, negative predictive value, and positive predictive value. The proportion of cows classified as high risk among the 4 algorithms ranged from 0.31 to 0.63, indicating that these approaches to SDCT could reduce antibiotic use at dry-off by 37 to 69% in the average US herd. All algorithms had poor agreement with IMI status, with kappa values ranging from 0.05 to 0.13. Sensitivity varied by pathogen, with higher values observed when detecting IMI caused by Streptococcus uberis, Streptococcus dysgalactiae, Staphylococcus aureus, and Lactococcus lactis. Negative predictive values were high for major pathogens among all algorithms (≥0.87), which may explain why algorithm-guided SDCT programs have been successfully implemented in field trials, despite poor agreement with overall IMI status. Removal of clinical mastitis criteria for each algorithm had little effect on the algorithm classification of cows, indicating that algorithms based on SCC alone may have similar performance to those based on SCC and clinical mastitis criteria. We recommend that producers implementing algorithm-guided SDCT use algorithm criteria that matches their relative aspirations for reducing antibiotic use (high specificity, positive predictive value) or minimizing untreated IMI at dry-off (high sensitivity, negative predictive value).

Cow-level risk factors for clinical mastitis in the dry period in cows treated with an internal teat sealant alone at the end of lactation

AIMS

To assess potential cow-level risk factors for clinical mastitis during the dry period in cows infused with internal teat sealant (ITS) alone at drying off, and associations with elevated somatic cell counts (SCC) at the first herd test, or clinical mastitis in the subsequent lactation.

METHODS

Ten herds reporting an incidence of clinical mastitis during the dry period of >3% amongst cows infused with ITS alone at the end of the 2018/2019 lactation were enrolled. Cow data collected included age, breed, milk yield and SCC at the last herd test before drying off, drying off date, clinical mastitis records during the dry period and in the subsequent lactation, and SCC at the first herd test following calving.

RESULTS

Overall, 2,401 cows were infused with ITS alone and 196/2,401 (8.2%) cows were diagnosed with clinical mastitis during the dry period. In the final multivariable model for risk of clinical mastitis there was an interaction between age and milk yield at the last herd test (p < 0.001), with cows aged >4 years and producing ≥10 L/cow/day having a higher risk of clinical mastitis than younger cows or those producing <10 L/cow/day. The risk of clinical mastitis was higher for cows dried off in the last week of lactation than for cows dried off >1 week before the end of lactation for the herd (p = 0.033). The percentage of cows with elevated SCC (>150,000 cells/mL) at the first herd test in the following lactation was higher amongst those diagnosed with clinical mastitis over the dry period compared with those that were not (p = 0.048). The hazard of clinical mastitis in the first 60 days after calving was also higher in cows diagnosed with clinical mastitis in the dry period than those that were not (p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE

The risk of clinical mastitis in the dry period following infusion of ITS alone at the end of lactation was associated with cow age, milk yield before drying off and timing of drying off. Cows diagnosed with clinical mastitis over the dry period had a higher risk of clinical mastitis and of elevated SCC in the subsequent lactation. This was an observational study so no causal inferences can be made, however herdowners and veterinarians should follow current recommendations regarding management of milk yield before drying off, and selection of cows for treatment with ITS alone.

Postcalving udder health and productivity in cows approaching dry-off with intramammary infections caused by non-aureus Staphylococcus, Aerococcus, Enterococcus, Lactococcus, and Streptococcus species

The objective of this prospective cohort study was to explore associations between intramammary infection (IMI) in late-lactation cows and postcalving udder health and productivity. Cows (n = 2,763) from 74 US dairy herds were recruited as part of a previously published cross-sectional study of bedding management and IMI in late-lactation cows. Each herd was visited twice for sampling. At each visit, aseptic quarter milk samples were collected from 20 cows approaching dry-off (>180 d pregnant), which were cultured using standard bacteriological methods and MALDI-TOF for identification of isolates. Quarter-level culture results were used to establish cow-level IMI status at enrollment. Cows were followed from enrollment until 120 d in milk (DIM) in the subsequent lactation. Herd records were used to establish whether subjects experienced clinical mastitis or removal from the herd, and DHIA test-day data were used to record subclinical mastitis events (somatic cell count >200,000 cells/mL) and milk yield (kg/d) during the follow-up period. Cox regression and generalized estimating equations were used to evaluate the associations between IMI and the outcome of interest. The presence of late-lactation IMI caused by major pathogens was positively associated with postcalving clinical mastitis [hazard ratio = 1.5, 95% confidence interval (CI): 1.2, 2.0] and subclinical mastitis (risk ratio = 1.5, 95% CI: 1.3, 1.9). Species within the non-aureus Staphylococcus (NAS) group varied in their associations with postcalving udder health, with some species being associated with increases in clinical and subclinical mastitis in the subsequent lactation. Late-lactation IMI caused by Streptococcus and Streptococcus (Strep)-like organisms, other than Aerococcus spp. (i.e., Enterococcus, Lactococcus, and Streptococcus spp.) were associated with increases in postcalving clinical and subclinical mastitis. Test-day milk yield from 1 to 120 DIM was lower (-0.9 kg, 95% CI: -1.6, -0.3) in late-lactation cows with any IMI compared with cows without IMI. No associations were detected between IMI in late lactation and risk for postcalving removal from the herd within the first 120 DIM. Effect estimates reported in this study may be less than the underlying quarter-level effect size for IMI at dry-off and postcalving clinical and subclinical mastitis, because of the use of late-lactation IMI as a proxy for IMI at dry-off and the use of cow-level exposure and outcome measurements. Furthermore, the large number of models run in this study (n = 94) increases the chance of identifying chance associations. Therefore, confirmatory studies should be conducted. We conclude that IMI in late lactation may increase risk of clinical and subclinical mastitis in the subsequent lactation. The relationship between IMI and postcalving health and productivity is likely to vary among pathogens, with Staphylococcus aureus, Streptococcus spp., Enterococcus spp., and Lactococcus spp. being the most important pathogens identified in the current study.

Randomized controlled non-inferiority trial investigating the effect of 2 selective dry-cow therapy protocols on antibiotic use at dry-off and dry period intramammary infection dynamics

Selective dry-cow therapy (SDCT) could be used to reduce antibiotic use on commercial dairy farms in the United States but is not yet widely adopted, possibly due to concerns about the potential for negative effects on cow health. The objective of this study was to compare culture- and algorithm-guided SDCT programs with blanket dry-cow therapy (BDCT) in a multi-site, randomized, natural exposure, non-inferiority trial for the following quarter-level outcomes: antibiotic use at dry-off, dry period intramammary infection (IMI) cure risk, dry period new IMI risk, and IMI risk at 1 to 13 d in milk (DIM). Two days before planned dry-off, cows in each of 7 herds were randomly allocated to BDCT, culture-guided SDCT (cult-SDCT), or algorithm-guided SDCT (alg-SDCT). At dry-off, BDCT cows received an intramammary antibiotic (500 mg of ceftiofur hydrochloride) in all 4 quarters. Antibiotic treatments were selectively allocated to quarters of cult-SDCT cows by treating only quarters from which aseptically collected milk samples tested positive on the Minnesota Easy 4Cast plate (University of Minnesota, St. Paul, MN) after 30 to 40 h of incubation. For alg-SDCT cows, antibiotic treatments were selectively allocated at the cow level, with all quarters receiving antibiotic treatment if the cow had either a Dairy Herd Improvement Association test somatic cell count >200,000 cells/mL during the current lactation or 2 or more clinical mastitis cases during the current lactation. All quarters of all cows were treated with an internal teat sealant. Intramammary infection status at enrollment and at 1 to 13 DIM was determined using standard bacteriological methods. The effect of treatment group on dry period IMI cure, dry period new IMI, and IMI risk at 1 to 13 DIM was determined using generalized linear mixed models (logistic), with marginal standardization to derive risk difference (RD) estimates. Quarter-level antibiotic use at dry-off for each group was BDCT (100%), cult-SDCT (45%), and alg-SDCT (45%). The crude dry period IMI cure risk for all quarters was 87.5% (818/935), the crude dry period new IMI risk was 20.1% (764/3,794), and the prevalence of IMI at 1 to 13 DIM was 23% (961/4,173). Non-inferiority analysis indicated that culture- and algorithm-guided SDCT approaches performed at least as well as BDCT for dry period IMI cure risk. In addition, the final models indicated that the risks for each of the 3 IMI measures were similar between all 3 treatment groups (i.e., RD estimates and 95% confidence intervals all close to 0). These findings indicate that under the conditions of this trial, culture- and algorithm-guided SDCT can substantially reduce antibiotic use at dry-off without negatively affecting IMI dynamics.

Evaluation of rapid culture, a predictive algorithm, esterase somatic cell count and lactate dehydrogenase to detect intramammary infection in quarters of dairy cows at dry-off

Our objective was to compare four tests to standard milk culture followed by MALDI-ToF in quarters of cows at dry-off. Cows (n = 432) were randomly selected from seven U.S. dairy herds already participating in a multi-site clinical trial in summer 2018. Aseptic foremilk samples were collected from quarters (n = 1728) two days prior to dry-off, and subjected to index and reference tests. The four index tests included rapid culture, a predictive algorithm, an esterase strip test measuring somatic cell count (SCC) and a cow-side lactate dehydrogenase (LDH) test. Rapid culture was performed by inoculating quarter milk samples onto a commercial rapid culture plate. Plates were evaluated by technicians after 30-40 h of incubation at 37 ± 2 °C. Quarters were classified as infected if any bacterial growth was observed. For the algorithm test method, all quarters were classified as infected if the cow met any of the following criteria: 1) any Dairy Herd Improvement Association (DHIA) test with a SCC > 200,000 cells / ml during the current lactation or 2) two or more clinical mastitis cases during the current lactation. Esterase-SCC and cow-side LDH tests involved adding milk to the test strip and reading for color changes. For esterase-SCC and cow-side LDH tests, low (≥250 cells / ml and ≥100 U / L) and high (≥500 cells / ml and ≥200 U / L) thresholds were used to classify quarters as infected or not. Composite samples (4 × 2 mL quarter-milk samples commingled) were also tested for rapid culture, esterase-SCC and cow-side LDH tests, such that if a composite sample was positive, then all quarters contributing to that sample were classified as infected. The reference test was traditional aerobic culture conducted in an accredited laboratory using MALDI-ToF for identification of isolates. Traditional culture was only conducted on quarter-milk samples, and consequently, IMI was always considered at the quarter-level. Unconditional logistic regression was used to estimate sensitivity (SE), specificity (SP), apparent prevalence, positive predictive values (PPV) and negative predictive values (NPV) for each index test. Cohen's Kappa (κ) was used to measure agreement between tests. Algorithm, esterase-SCC and cow-side LDH tests had poor agreement with the reference test (κ ranging from 0.01 to 0.12), while rapid culture had fair agreement (κ = 0.28). No test had concurrently high SE and SP. Negative predictive values were moderate to high for all tests.

Evaluation of 4 different teat disinfection methods prior to collection of milk samples for bacterial culture in dairy cattle

The first objective of this study was to determine whether differences would occur among teat end preparation techniques with regard to potential contamination of milk samples collected for bacterial culture. The second objective was to determine whether differences would be detected in genus or species of bacteria isolated from samples collected using the various methods as well as from contaminated or uncontaminated samples. Mammary quarter foremilk samples were collected from lactating dairy cattle at the University of Missouri Foremost Research Dairy Farm (Columbia). Four different teat end preparation methods were used to compare contamination rates in milk samples. Sampling techniques used before milk collection included (1) no preparation, (2) pre-milking disinfection and single-use towel drying of teats only, (3) scrubbing of the teat end with alcohol only, and (4) pre-milking disinfection, single-use towel drying, and scrubbing of the teat end with alcohol. Milk was plated on Columbia blood agar. Cultures were read at 48 h, with the number of morphologically different bacterial colony types quantified and isolated. Isolates were identified using MALDI-TOF mass spectrometry. Median numbers of colony types were compared among groups using Kruskal-Wallis ANOVA with post-hoc pairwise comparisons, and proportional data were compared using the chi-squared test. A total of 168 cows, including 665 quarters, were sampled, and 1,614 isolates resulted. Analysis with MALDI-TOF identified 29 unique genera and 81 unique species among the samples. More contaminated samples occurred in groups 1 and 2 compared with groups 3 and 4. Group 3 had more contaminated samples than group 4. The majority of Pseudomonas spp. isolates were identified within group 2. When applying previously described niches to Staphylococcus spp., environmental species were more likely to be identified among contaminated samples, whereas host-adapted species were more likely to be identified among uncontaminated samples. These data confirm that scrubbing the teat end with alcohol after pre-milking disinfection with an iodine-based teat disinfectant and drying of the teat minimizes contamination of the milk sample.

Use of different somatic cell count cut-points to define intramammary infection at drying off in dairy cows from a herd with a high somatic cell count

Aims: To assess the use of different cut-points based on individual cow somatic cell counts (SCC) to define cows with intramammary infection (IMI) at drying-of, in a herd with a high mean bulk tank SCC. Methods: Results for SCC from four herd tests during lactation and bacterial culture of milk samples collected before drying-off were obtained for 139 cows from a herd with an average bulk milk SCC of >300,000 cells/mL over the final 4 months of the 2006/07 lactation. Based on culture results, cows were defined as being infected with a major (Staphylococcus aureus, Streptococcus uberis or Nocardia spp.) or any pathogen. Receiver-operator characteristics (ROC) curves were used to determine optimum cut-points for maximum, average and last herd test SCC, for predicting IMI. Multivariable logistic regression models were used to determine which variables were associated with IMI, and the sensitivity (Se), specificity (Sp) and positive predictive value (PPV) were determined for different cut-points. Results: At the cow level, 75/139 (54.0%) cows had IMI with a major pathogen and 123/139 (88.5%) with any pathogen. A SCC ≥150,000 cells/mL at ≥2 herd tests and a SCC ≥299,000 cells/mL at the last herd test, for cows aged ≥4 years, were associated with IMI with a major pathogen at drying-off (p<0.05). A SCC ≥150,000 cells/mL at ≥2 herd tests was associated with IMI with any pathogen at drying-off (p<0.001). A cut-point of ≥150,000 cells/mL at any herd test had the highest Se (0.97 and 0.94), but the lowest Sp (0.19 and 0.44) and PPV (0.58 and 0.93) for infection with major and any pathogens, respectively. A cut-point of ≥150,000 cells/mL at ≥2 herd tests doubled the Sp and increased the PPV without large decreases in test Se for infection with either a major or any pathogen. Conclusions and clinical relevance: In this herd with a high bulk milk SCC, use of a cut-point of a SCC ≥150,000 cells/mL at any herd test to define IMI would be appropriate, where the goal at drying-off is to ensure that cows infected with any pathogen receive antimicrobial treatment. Where the goal is to reduce the use of antimicrobial dry cow therapy in uninfected cows while limiting the number of infected cows not being treated, use of a cut-point of SCC ≥150,000 cells/mL at ≥2 herd tests to define IMI may be more appropriate.

Effect of using internal teat sealant with or without antibiotic therapy at dry-off on subsequent somatic cell count and milk production

The objective of this study was to assess the effect of treating cows with teat sealant only compared with antibiotic plus teat sealant at drying off on weekly somatic cell count, potential intramammary infection, and milk production across the entire subsequent lactation. In 3 research herds in the south of Ireland, cows with SCC that did not exceed 200,000 cells/mL in the previous lactation (LowSCC) were randomly assigned to 1 of 2 treatments at drying off: internal teat sealant alone (ITS) or antibiotic plus teat sealant (AB+ITS). Cows with SCC that exceeded 200,000 cells/mL in the previous lactation were treated with AB+ITS and included in the analyses as a separate group (HighSCC). Weekly individual animal composite SCC records were available for 654 cow lactations and were transformed to somatic cell scores (SCS) for the purpose of analysis. Data were divided into 3 data sets to represent records obtained (1) up to 35 DIM, (2) up to 120 DIM, and (3) across the lactation. Foremilk secretions were taken from all quarters at drying off, at calving, 2 wk after calving, and in mid-lactation and were cultured to detect the presence of bacteria. The LowSCC cows treated with ITS alone had higher daily milk yield (0.67 kg/d) across lactation compared with LowSCC cows treated with AB+ITS. The LowSCC cows treated with ITS alone had higher SCS in early, up to mid, and across lactation compared with LowSCC cows treated with AB+ITS. We detected no difference in weekly SCS of LowSCC cows treated with ITS alone and SCS of HighSCC cows. The least squares means back-transformed SCC across lactation of the LowSCC cows treated with ITS alone, LowSCC cows treated with AB+ITS, and HighSCC cows were 41,523, 34,001, and 38,939 cells/mL respectively. The odds of LowSCC cows treated with ITS alone having bacteria present in their foremilk across lactation was 2.7 (95% confidence interval: 1.91 to 3.85) and 1.6 (1.22 to 2.03) times the odds of LowSCC cows treated with AB+ITS and of HighSCC cows treated with AB+ITS, respectively. In this study, Staphylococcus aureus was the most prevalent pathogen isolated from the population. Recategorizing the threshold for LowSCC cows as ≤150,000 cells/mL or ≤100,000 cells/mL in the previous lactation had no effect on the results. The results indicate that herds with good mastitis control programs may use ITS alone at dry-off in cows with SCC <200,000 cells/mL across lactation with only a small effect on herd SCC.